Gap between land and sky in world background (cycles)
This is a deficiency of your ocean surface not curving away from you "to infinity" the way a real ocean would. Since it's instead a finite plane, it's possible to get above it and see through the area where an infinite plane should've occupied. Some possible workarounds:

1) Make the plane bigger and lower the camera angle. You may be able to minimize the gap to the point that it's not an issue.

2) If you're committed to the higher camera angle, you can use the mist pass as suggested by cegaton to cover up the horizon. You can use a mask or gradient texture in the compositor to cancel the mist pass effect in the upper part of the frame, and thus expose the sky above the gap